This invention relates in general to pressure vessels and, more particularly, to process for fabricating layered heads and shells for pressure vessels.
The typical pressure vessel has a cylindrical shell and heads that close the two ends of the shell. The shell and heads are manufactured as separate components and are thereafter joined together by welding to produce a unitary structure. To enable the heads to better withstand elevated pressures, they are usually somewhat dish-shaped, having their concave surfaces presented inwardly toward the interior of the vessel. Indeed, in many vessels, particularly those designed to withstand high pressures, the heads are hemispherical.
It is generally recognized that pressure vessel walls composed of multiple layers are superior to thick single walls in many respects. For example, the individual plates of a layered wall, generally speaking, have better metallurgical properties than thick single walls since they are subjected to greater rolling at the mill. As a consequence, a layered vessel is usually safer than a solid wall vessel of equivalent wall thickness. Similarly, because of their better metallurgical properties, the individual layers of layered walls do not tend to laminate as often occurs with solid walls. Also, in layered vessels it is possible to vary the metal alloy from layer to layer, thus enabling an expensive corrosion resistant liner to be used with less expensive, yet stronger, surrounding layers. While a variety of thick steel plate clad with various corrosion resistant alloys is available from steel mills, such plate is expensive. Moreover, thin layers are relatively easy to shape, but this is not the case with the heavy steel plate used in solid wall vessels. Thus, layered walls can be manufactured in greater thicknesses than solid walls. Aside from that, the individual layers that comprise the walls of a layered vessel, upon being welded together, tend to shrink as the welds which join them solidify and cool, thereby placing the vessel walls in a state of precompression. This is desirable since the elevated pressures within the vessel create tensile forces in the vessel walls. In contrast, solid wall vessels are normally heat treated to relieve them of stress concentrations, and therefore do not exist in a state of precompression.
Heretofore different procedures have been developed for fabricating cylindrical shells from multiple layers, one highly successful procedure being set forth in U.S. Pat. No. 3,478,784. Heads, by reason of their compound curvatures, are not so easily fabricated in multiple layers, and as a consequence most heads are still of solid wall construction. Thus, to a large measure, the pressures to which present pressure vessels may be raised are limited by the heads at their ends.
One of the major problems encountered in the fabrication of both layered heads and shells is that of obtaining metal-to-metal contact between adjacent layers so that the overlying layer effectively reinforces the underlying layer. Without this type of contact the underlying layer is often stressed significantly before the overlying layer effectively backs it and assumes some of the load imposed by the pressurized contents of the vessel. Unless metal-to-metal contact exists between adjacent layers, the so-called loose liner problem is repeated at every interface at which such contact is lacking.